The morphological differentiation between small-cell and non-small-cell lung cancer has great prognostic and therapeutic significance for the patient. Malignant lung tumors are now classified according to the new 1999 WHO/IASLC classification of lung and pleural tumors. The variant of heterogeneously differentiated "combined small-cell carcinoma" can be distinguished from classical small-cell carcinoma, whereas the subtype of "intermediate cell carcinoma" is no longer used. Together with "large-cell neuroendocrine carcinomas" and typical or atypical carcinoid tumors, small-cell lung cancers are currently histogenetically categorized as neuroendocrine lung tumors. In contrast to large-cell neuroendocrine carcinoma, the immunohistochemical demonstration of neuroendocrine differentiation is not a prerequisite for the diagnosis of small-cell lung cancer. Although electron-microscopical, immunohistochemical, and molecular-biological findings have considerably increased our understanding of the pathogenesis and progression of malignant lung tumors, routine pathological-anatomical diagnostics are still decisively based on light-microscopical evaluation of tissue samples.